Method of determining scale of aerial photographs for map making



"I". ELIEL METHOD OF DET RMINING SCALE OF AERIAL PHOTOGRAPHS FOR MAPMAKING Filed April 12, 1940 2 Sheets-Sheet l Z50 500 I I June 15, 1943.L. T. ELxEL 321,902

' METHOD OF DRTERMINING SCALE OF AERIAL PHOTOGRAPHS FOR MAP MAKING FlledAprll l2, 1940 2 Sheets-Sheet 2 Patented Jane 1s, 1943 METHOD OFDETERMINING SCALE OF AERIAL PHOTOGRAPHS FOR MAP MAK- ING Leon T. Eliel,Pasadena, Calif., assignor to Fairchild, Polley & Elel, Inc., LosAngeles, Calif., a corporation of California Application April 12, 1940,Serial No. 329,312

6 Claims.

My invention relates to aeronautics, with special reference to aerialnavigation and mapping, and, more particularly is directed to a methodand apparatus for ascertaining during flight the elevation above groundof an aircraft.

Approximation of terrain clearance or the vertical distance from anaircraft to the terrain below is sufficient when safety alone is theconsiderationy as when a pilot is forced to fly blind over unidentifiedterrain; but accurate knowledge of the vertical distance is essentialfor calculating the scale of aerial maps and is essential also inwartime for achieving accuracy in bombing objectives from great heights.

The older and most widely practiced method of arriving at terrainclearance of an aircraft is to subtract the known altitude above sealevel of the terrain from a barometric reading of altitude on theaircraft. This method is of limited service in blind flying where thepilot is sure of his course' and position on the course, but it isValueless if the blind flying is over unknown terrain; for eX- ample,when a pilot is carried off his course and lost in a storm. Even underthe most favorable conditions the method may not be suilicientlyaccurate for mapping because atmospheric pressure changes with theweather and because barometers and aneroids at best indicate onlyapproximate distances above sea level.

Because it is necessary to identify terrain points below the aircraftand then to ascertain the altitudes of the identified terrain points,this older method often involves prolonged map study even when theterrain is familiar to the pilot. the stress of wartime ilying, a pilotmay not have opportunity during flight for adequate study of maps athand, and often enough maps of enemy terrain are not available. Again inpioneer aerial mapping over unexplored terrain, the absence of maps Withadequate altitude data precludes theA tolerable when the method is to beused for safety in flight only, but it prevents widespread applicationof the method to aerial photography since photographic mapping iscommonly practiced at flight levels well over 10,000 feet terrainclearance and Wartime ying for mapping or bombing Under All) enemyterrain is usually conducted far above 4000 feet terrain clearance.

The general object of my invention is to provide a method ofascertaining terrain clearance accurately and quickly from any height bya physical procedure without the necessity for any maps or other data ofthe terrain below. Broadly described, my method is characterized by thesteps of dropping an object to the terrain from the elevated level ofthe aircraft, timing the fall of the object, and computing the verticaldistance involved from the duration of the fall.

One group of objects of my invention relate to the apparatus itselfemployed for ascertaining the terrain clearance. My objectives includedependability and simplicity of operation, accuracy with minimum errorsfrom mechanical sources, largely automatic operation with minimum burdenon the pilot. prompt visual indication of the elapsed time of a fall,and permanent recording of the elapsed time for subsequent use.

Another group of objects in mind relates to the practice of my inventionin map-making procedures. One of these objects is to derive terrainclearance data at the time of making an aerial photographic exposure,the data being available for subsequent calculations to arrive at thescale of the photographic image. A further object in the map-makingprocedure is to locate on the terrain the precise point at which terrainclearance is measured so that the identifled point may be used as analtitude reference. For example, other altitudes on the terrain may bederived from comparison with such a reference point by stereoscopicmethods'. With reference to this last object my invention ischaracterized by the concept of employing a falling object that makes avisual signal upon impact with the earth and then photographing theterrain to include the visual signal.

Other objects and advantages of my invention will be apparent from themore detailed description tov follow, taken with the accompanyingdrawings.

In the drawings which areto be taken as illustrative o'nly:

Fig. l is a representation of one form of my apparatus, therepresentation being partly diagrammatic and partly in side elevation;

Fig. 2 is a transverse section of a portion of my apparatus taken asindicated by the line 2-2 of Fig. 1;

Fig. 3 is an enlarged view in side elevation of the preferred form offalling object employed in the practice of my invention;

Fig. 4 is a face view of a record film produced in the operation of theapparatus, the hn being shown as a positive for convenience ofillustration;

Fig. 5 is an aerial photograph revealing a visual signal produced byimpact of the falling object with the earth;

Fig. 6 is a view of the same character as Fig. 1 indicating theconstruction of a second form of my invention;

Fig. '7 is a face View of a record strip produced Vby the apparatus ofFig. 6; and

Fig. 8 is a view of the' same character as Fig. 6 showing a thirdembodiment of my invention.

My invention may be practiced in any manner that affords an opportunityto time the fall of an object from an airplane to the terrain below. Forexample, a simple procedure may consist of the pilot dropping somesuitable object by hand, visually following the course of the dropped object, timing the fall by means of a stop watch, and then calculating thelength of the fall by the observed duration. I prefer, however, toemploy by means of suitable apparatussome timing pro cedure that islargely automatic because such a procedure places little or no burden onthe operator and reduces the likelihood of error. Fig. 1 indicates theconstruction of an apparatus for this purpose.

The principal elements of the apparatus or system depicted by Fig. 1are: an object generally designated IO that is suitable for dropping tothe earth; a release mechanism generally designated Il for dropping theobject; a radio receiver represented by a conventional circuit generallydesignated I2; a timing mechanism or indicator generally designated I3;and an aerial camera I4.

What my be called the free-falling char-v acteristic of the object I0must be knownior accurate results. In other words, air resistancedetermined by the configuration of the object I0 will cause the objectto fall to the terrain at a slower rate than the object would fall in avacuum. The free-falling characteristic may be expressed in a formulafor the rate of fall of the object or may be inherent in a table orcurve that is derived either empirically or by calculation.

The one essential function of the object I0 is simply that it may beadapted to fall to the earth in a predictable manner. In the preferredform of my invention, however, the object I0 is adapted to transmitradio waves continuously during its fall to permit automatictiming, andthe object is furthermore adapted to produce be provided. 'I'heconstruction of such a transmitter of small weight and' small volumepresents no diillcult problems and may be patterned largely on thetransmitters now employed on small balloons for automaticallybroadcasting meteorological data. The nose of the timing bomb I8contains a body 22 of mildly explosive material. To favor the formationof a readily visible smoke cloud when the bomb strikes the earth, abottle 23 of` some suitable chemical may be placed in the bomb to bebroken and scattered by the explosion of the material 22. A detonator 24surrounded by the explosive material 22 is adapted to be set off by asuitable 'plunger 25 at the nose of the bomb, the plunger being normallyheld in extended position by a suitable spring 26. A manually removablepin 21 may be inserted in a suitable diametric bore in the plunger 25 toprevent accidental detonation, the pin being manually removed when thebomb is placed in the release mechanism II.

The bomb release mechanism II may be any suitable device for suspendingthe bomb I0 from an aircraft in a releasable manner. The particularrelease mechanism II shown in Fig. l

`is carried by a downwardly extending bar 30 that is rigid with theaircraft. The bar tersome type of signal readily visible from theaircraft to indicate the point of impact with the ground. While such avisual signal may be produced mechanically, I prefer simplyto produce asmoke `cloud by combustion when the object strikes the earth. Thepreferred form o f the object I0, then, shown in Figs. 1 and 3, isadapted for explosive combustion and may be conveniently referredto as atiming bomb.

The timing bomb I0 is of streamline configuration with trailing fins ortail members I5 and an eye IE at its trailing end for suspension in therelease mechanism H. The interior of the bombI houses ay radiotransmitter II anda battery I8 for the transmitter, the transmitterbeing adapted to energizeY a transmitting aerial I9 in the form of awire extending from an insulatorv bushing 20. For controlling theoperation of the transmitter a suitable switch .2| may minates in afinger 3l that cooperates with a hook 32 in releasably retaining thetiming bomb, the hook engaging the previously mentioned eye I6. The hook32 is an extension of a manual release lever 33 that is mounted on thebar 30 by a suitable pivot 34.4 To serve as a stop to prevent the eye I6from following the lateral f opening movement ofthe hook 32, anysuitable stop means may be employed such as a crossbar 3S just below thehook K32.

The responsive energy of the radio receiver I2 is delivered to a pair ofoutput wires 38 and 39 that terminate 'in the timing device I3. Thefunction of the timing device I3 may be to indicate at once for thebenefit of the pilot the falling interval of a timing bomb, or may beto.

make a permanent record of the elapsed time. or may be Iboth to indicateand record the time. The particular timing device I3 shown in Figs. 1and 2 is designed primarily for map-making purposes and makes a recordof the time interval without simultaneously indicating the time to theoperator.

Such a timing device may consist of an ammeter 4I! energized by theradio output wires 38 and 39 in combination with means to record on amoving strip the movements of 'a pointer or hand 4I that is responsiveto the ammeter. The ammeter is housed in a dark-box 43 with the pointer4I disposed across a slit 44 in the darkboX, and a lamp 45 directs lightinto the slit 44 to cast the shadow of the ammeter pointer 4I onto aphotographic lm 46. The lm 46 is wound onto a spool 48 from a spool 49and is held in an exposure plane close to the pointer 4I by a pair ofspaced rolls 50 and 5I. The movement of the lm is derived from aspring-driven motor 52 that is frictionally connected with the Spool 48and positively connected with the roll 5I, the roll 5 I being adapted torotate at constant speed and 4 having sprocket teeth to engage sprocketholes `53 in the lm (Fig. 4 Conveniently lo catedfor the operator is acontrol plunger 54 for the spring-driven motor 52, which plunger may beadapted to release the motor for operation when pushed inward and tostop the motor when pulled outward.

Since llm fmay change in length, especiallyl during development, meansmay be provided to record time intervals on the i-llm whereby timerepresented by the lm movement may be accurately ascertained regardlessof changes in the film length prior to time calculation. For thispurpose I provide a rotary disc 55 overlapping an end portion of thelight slit 44, the rotary disc being driven by a spring motor 56 at aconstant rate of one revolution per second. Normally, the disc shortensthe effective length of the light slit 44, but once each second aperipheral notch in the disc registers with th'e light slit to unmaskthe light slit to full length. As a result, the light pattern of thepositive film in Fig. 4 is a wide band 58 with tooth-like projections 59representing intervals of one second. By measuringT time on the film interms of these space intervals any error that might arise from change inlength of the film is obviated.

For automatic operation of the system in the determination of the heightof the aircraft from terrain, the timing device I3 should have someoperative relationship to the release mechanism II, since the timinginterval begins with the release of the timing bomb. The requiredrelationship may be achieved simply by placing an auxiliary battery 60and a switch 62 in parallel with the ammeter 40 across the output wires38 and 39 and mechanically connecting the switch 62 with the releasemechanism II. As indicated in Fig. l the battery 60 is connected to thewire 38 by a wire 53 and on the other side is connected to one terminalof the switch 62 by a wire 64, the other terminal of the switch beingconnected to the wire 39 by a wire 65. The dotted line 66 representssome suitable mechanical connection between the release hook 32 and theswitch 62. It is contemplated that the operator will swing the releaselever 33 downwardly in a vigorous manner to release the timing bomb Iand that in the course of such release movement the oper ativeconnection 6G will cause the switch 62 to close momentarily and thenopen.

The operation of this first form of my invention may be readilyunderstood by referringto Fig. 4 showing a portion of the film 45 thathas been developed to bring out a record of a fallingr interval. Forconvenience of illustration in black and white the lrn 4E in Fig.'4 isdepicted as a positive print instead of a negative that would be used inpractice.

Since the lm is positive, the light band 58 with its projections 59 iswhite and the shadow of the ammeter pointer 4I is represented by a blackline I0 on the white band. When the ammeter 4U is deenergized, thepointer 4I rests at a position near one end of the light slot 44 andtherefore causes the black line to be traced on the light band 58 nearone edge of the light band. In Fig. 4 wherein the record is to be readfrom left to right, the end of the recordshows the black line 'I0dropped to a low lcvel at 'II representing the deenergized position ofthe ammeter pointer.

In a typical procedure the operato-r places one of the timing bombs I0in the release mechanism operator releases the timing bomb Ill by avigorous opening movement of the release lever 33, additional currentfrom the auxiliary battery 60 momentarily surges through the ammeter 40to swing the ammeter pointer to a relatively high level, and as a resultthe line I0 on the developed lm swings to a peak 13, and then returns tothe intermediate level I2 for the duration of the fall.

II, initiates operation of the transmitter I'I in the timing bomb bymanipulating the switch 2| and tunes in the receiver I2 on the aircraft.After the receiver reaches a normal level of output through the ammeter40, the operator presses the control plunger 54 to start movement of thelm 48, whereupon the ammeter causes the black line 10 to be traced at anintermediate level on the iilm, as indicated at I2 in Fig. 4, When thethe iilm 46 it is to be measured from the point at which the line 'I0initially rises to the peak 'I3 to the point at which the line I0initially swerves to make the abrupt drop 14, it is apparent thatneither the sharpness of the peak 'I3 nor the abruptness of the drop 'I4is a factor in the accuracy of the time reading. It is apparent, then,that the switch 62 may be closed long enough to form a plateau insteadof the peak I3 and may in fact remain closed throughout the duration ofthe fall since the result of keeping the switch 62 closed would bemerely to have the drop '14 appear at a higher level.

When the film is subsequently developed at the termination of a iiight,the duration in seconds of the fall of the object I0 may be readdirectly from the film 46 by employing as measuring units the intervalsindicated by the lateral projections 59 of the light band 58, and fromthe time so derived the vertical distance traversed in the fall of thetiming bomb may be readily computed. A shorter procedure consists inapplying to the developed lm 46 a scale 15 calibrated to the spacing ofthe light band projections 59 and graduated in terms of verticaldirection in feet. As

indicated in Fig. 4 the scale 'l5 is placed with one end at thelongitudinal point of the lm 46 where the line I0 initially rises to thepeak 13 and the distance in feet is read from the point on the scaleopposite the longitudinal point of the lm at which the abrupt drop 14 ofthe line 10 appears. The fall of the timing bomb I0 represented by thelm record in Fig. 4 is revealed by the scale I5 to cover a verticaldistance of 10,800 feet.

A further procedure in the practice of my invention as applied to aerialphotography may be understood by considering an aerial photograph 18shown in Fig. 5. Since the focal length of the camera I4 employed on theaircraft is known, only the vertical distance from the camera to a pointon the terrain is required to compute the scale of the photographicimage at this point. Usually it is desirable to produce'ultimately anaerial photograph or other map representation at some standard scalewhich may differ from the scale of an original photograph. Knowledge ofthe scale of a point on a photograph is valuable in many types of mapcompilation. If a simple mosaic is being made, a ratio factor may bederived from this known scale. If radial control is to be utilized, theknown scale is very useful, particularly in the application of theslotted templet method set forth in the Collier Patent No. 2,102,612 andin the Eliel, et al. Patent No. 2,180,406.

If the practice of my invention is to be carried farther to ascertainrelative altitudes cf points on the photographed terrain, it isnecessary to know at what point of the terrain the elevation aboveground of the aircraft is taken.

The photograph 10 discloses a signal cloud 19 produced by explosion uponimpact of a timing bomb and therefore indicates the point on thephotographed terrain to which the vertical distance represented by thetiming record is to be applied. The scale of a random terrain point is agreat help in compiling a contour map, and xes the horizontal and therelative vertical scale, although to know absolute height above setlevel some other means must be employed. It is not necessary to know theabsolute height above sea level of terrain to prepare adequate maps formilitary purposes, for example. To ascertain the altitudes of otherpoints on the terrain relative'to the impact point 19, the images ofsuccessive overlapping photographs are correlated to produce astereoscopic image revealing relative altitudes. By well known mappingmethods, such a stereoscopic image may be employed to calculate relativealtitudes and to locate contours accurately, the scale of thestereoscopic image being known.

In the practice of a second form of my invention represented by Figs. 6and 7, I propose to afford the pilot or operator an immediate reading ofthe falling time of a bomb and to make the timing and recording of afall fully automatic, leaving to the pilot or operator merely the taskof handling the successive timing' bombs to b'e dropped from theaircraft.

The principal elements of the system shown in Fig.. 6 are: abomb-release mechanism 8|; a mechanical switch generally designated 82associated with the release mechanism; an output circuit of a radioreceiver on the aircraft, the output circuit being represented by wires83 and 84; a relay 85 in series with wires 83 and 84; a main timingcircuit represented by wires 86 and 81'; an auxiliary timing circuitrepresented by wires 88 and 89; a clock 90 for timing the fallingintervals; and a camera 9| for photographing the clock. The releasemechanism 8| is substantiallythe same as the previously describedrelease mechanism being manually operated by a lever 92 corresponding tothe lever 33 of Fig. l.

The mechanical switch 82 that is associated with the release mechanism8| includes a ilexible switch arm 95 having two spaced and insulatedbridging contacts 96and 91. When a timing bomb is engaged by the releasemechanism 8|, the eye |6.by which the bomb is engaged presses upward toflex the switch arm 95 upward and thereby hold the switch 82 open. Whena timing bomb is released by the mechanism 8| the switch arm 95 returnsto its normal closed position at which the bridging con'- tact 96interconnects the wires 86 and 81 of the main timing circuit and thebridging contact 91 interconnects the wires 88 and 89 of. the auxiliarytiming circuit..

The relay 85 includes a switch member 99 in the main timing circuit thatis normally open when the relay is deenergized. It is apparent that themain timing circlit is closed only when the relay 85 is energized andsimultaneously the mechanical switch 82 is closed.

The camerav 9| |00 operated by a solenoid I 0| and the shutter isadapted to open momentarily for a photographic exposure whenever thesolenoid operates in one direction upon energization or operates in theopposite direction upon deenergization. The camera 9| is also providedwith a spring-actuated motor |02 for winding film between exis equippedwith a shutter directions of operation of the mechanism controlled bythe lm solenoid |03 and by furtherV providing for retardation. in theoperation of the iilm solenoid in both directions. For such retardationI may employ an apertured bellows |08 of a familiar type that isoperatively connected to the lm solenoid |03 to exert a dashpot effect.

The clock may have a circular scale |09' having one hundred graduationsto represent seconds with respect to a clock hand ||0 and to representhundredths of a second with respect to a longer clock hand the hand ||0advancing one graduation per second and the longer hand making acomplete revolution in a second. The face of the clock appears in thephotographic lm ||2 shown in Fig. 7. Conceivably, the clock scale mayexpress feet in terrain clearance. The clock preferably is provided witha continuously running springdriven motor (not shown) energized by amanual key ||3, the hands being adapted to be actuated by the motor onlywhen a clutch IllA of the clock is in engagement. The clutch ||4 isoperatively connected to a solenoid Illa and is normally out ofengagement with the solenoid ||4a deenergized. Whenever the solenoidIlla is energized it holds the clutch ||4 in engagement to cause thehands ||0 and to be advanced by the clock motor. In my preferredconstruction the clock hands are adapted to maintain whatever positionthe hands attain at the end of .a timing interval, but the hands arebrought back to their zero positions upon subsequent depression of aclearing plunger ||5 at the top of the clock.

The shutter solenoid |0|, the lm solenoid |03, and the clutch solenoid||4a are all arranged in parallel in the main timing circuit. Thenecessary connections include three wires branching from the maincircuit wire 81, a first wire H6 to the shutter solenoid, a second wire||1 to the film solenoid. and a third wire 8 to the clutch solenoid||4a. A wire 9 from one terminal of a battery |20 has threecorresponding branches, wires |2|, |22, and |23 leading respectively tothe shutter solenoid, the lm solenoid, and the clutch solenoid. Tocomplete the main timing circuit, a wire |24 interconnects the battery|20 and the switchrmember 99 of the relay 85. i

Preferably, some means is included to indicate visually' to the operatorwhen the main timing circuit is energized. Such means may comprisesimply a signal lamp |25 in a shunt across the wires-81 and 9. 1

Thewire 88 of the auxiliary timing circuitV extends from themechanicalswitch 82 to the wire |-|9 for communication with one side ofthe battery |20, and the other wire 89 of the auxiliary circuit extendsfrom the switch 82 to what may be termed a masking solenoid |20,

other side of the battery |20. The solenoid |28, which is equipped Withan apertured belolws |30 for delayed action, has two functions, first,to actuate the clearing plunger ||5, and, second, to actuate some meansfor indicating when energization of the main timing circuit is caused bythe release of a timing bomb and therefore represents an interval oftime upon which an altitude calculationl is to be based. The first oftheseA functions may be provided by mounting on the movable part of thesolenoid |28 a finger |32 positioned to depress the clearing plunger ||5at the end of a deenergization movement of the solenoid. The secondfunction may be achieved, for example, by mounting a signal member |33on the movable part of the solenoid |28 to move upward with energizationof the solenoid and downward when the solenoid is deenergized. In mypreferred arrangement the signal member |33 is a metal plate paintedblack to serve as a mask for a stationary white plate |34 mounted on theclock 90.

When the operator places a timing bomb ||l in engagement with therelease mechanism 8| he thereupon causes the switch arm 95 to be flexedupward to open the main timing circuit and the auxiliary timing circuit.Considering the presence of a timing bomb in the engagement mechanism 8|as normal, it is apparent that normally both the main timing circuit andthe auxiliary timing circuit are open and all four of the solenoids aredeenergized. Before releasing a timing bomb, the operator starts thetransmitter in the timing bomb and tunes in the receiver on the aircraftwith the result that the f.

switch member 99 of the relay 85 closes. No electrical effect is causedby the energization of the relay at such time because the mechanicalswitch 82 remains open.

When the operator swings the release lever 92, the release of the timingbomb permits the flexible switch member 95 to move to closed positionand instantly both timing circuits are closed. The immediate results ofthe release o'f the tlming bomb include the energization of the signallamp |25, the engagement of the clock clutch |4, and the operation ofthe solenoid |0| to take a picture of the clock 90, the clock-beingshown at zero time or close t'o zero time and the signal member |33being in its lower position to mask the white plate |34. The latereffects of simultaneously closing the two timing circuits are theadvancing of the photographic film by the retarded operation of the filmsolenoid |03 and the elevation of the signal member |33 to unmask thewhite plate |34, the delayed actions being such that the film in thecamera is stationary at the new position and the signal member |33 is atits uppermost position within five or six seconds after the two circuitsare energized.

When the falling timing bomb is demolished by impact against the earthwith consequent destruction of the transmitter therewith, output fromthe radio receiver on the aircraft through the relay 85 ceases withconsequent opening movement of the switch member 99 and deenerthe clockremain xed after the main circuitis gization of the main timing circuit,the secondary might be made accidentally.

broken so that the pilot may ascertain the time duration of the fall ata glance and derive the vertical distance of the fall directly from theclock scale if the scale is in feet, or by calculation, or by referringto a time table computed on the free-falling characteristic cf the typeof timing bomb employed.

When the pilot places a succeeding timing bomb I0 in the releasemechanism 8|, thereby flexing the switch member 95 upward, the auxiliarytiming circuit is thereby opened with consequent deenergization of themasking solenoid |28. After a delay of three or four seconds the signalmember |33 reaches its lowermost position to mask the white plate |34and simultaneously the nger |32 depresses the clearing plunger ||5 torestore the clock hands to zero position.

It is to be noted that in the above sequence of operations the soletaskrequired of the operator is to manipulate the release mechanism 8|,the required successive photographic exposures for ascertaining timeintervals being taken automatically. The only precaution required isthat the operator defer placing a ltiming bomb in the release mechanismuntil after the indicating lamp |25 goes out and then defer release ofthe bomb for approximately live seconds to permit the delayed action onthe part of the masking solenoid..l

The strip of developed film ||2 in Fig. 7, which is shown as a positiveprint for convenience of illustration, has an upper image |36 that showsthe clock 90 at zero position with the signal member or mask |33 in itslower position. A second image |31 immediately below the image |36 showsthe clock with the hands advanced to represent a substantial timeinterval and also shows the signal member |33 in its upper positionunmasking the white plate |34.

The signal plate or mask |33 is included in the arrangement to identifypairs of successive images on the record film that represent fallingintervals of timing bombs and to distinguish such significant pairs ofimages from images that If an operator starts the transmitter of atiming bomb before placing the bomb in the release mechanism, the relaymay respond by closing the main signal circuit, a first accidentalphotograph being taken when the main circuit is closed and a secondaccidental photograph being taken when subsequently the operator placesthe transmitting bomb in the release mechanism. The first accidentalimage would show the mask at its upper position and probably would showthe hands of the clock at advanced positions. The succeeding accidentalimage Vwould show the hands of the clock at still further advancedpositions and would again show the mask in its upper position. It isgenerally possible to identify the significant images on a developedfilm solely by comparing the various images with reference to thepositions of the clock hands, but the selection of the significantimages is greatly facilitated by also noting the. positions of the maskin the various images. Confusion can arise only if the operator places atiming bomb in the release mechanism or otherwise opens the mechanicalswitch 82 while a timing bomb is falling through the air. This source ofconfusion cannot arise if the operator waits through each falling perioduntil the lamp |25 goes out. Otherwise, the image of a clock at zerowith the mask down, followed by an image with the clock advanced and themask up may be depended upon as representing the fall of a timing bomb.

The system represented by Fig. 6 may be em- .ployed by a pilot toexplore the prole of terrain traversed by the ship when the pilot is:flying blind. To explore the prole of the terrain the pilot holds hisship toa given altitude as determined by his aneroid and releasessuccessive timing bombs, say, at one minute intervals; Without takingtime to calculate the vertical air distances involved, he may simplynote the relative magnitudes of the time readings to deduce therefromthe prole of the underlying terrain. progressively increasing fallingintervals would clearly indicate sloping of the terrain downward in theline of ight; progressively decreasing readings would denote theopposite; constant values would indicate relatively flat terrain or abody of water; and irregular sequence would clearly suggest brokencountry as well as positively indicate land as distinguished from Water.If the readings are jotted down and the ground speed estimated, theoperator may go so far as to derive the actual slope of the terrain. Inany case a series of successive readings will give a denite clue to thecharacter of the terrain and the clue may be suilicient guidance for theoperator to change his course intelligently.

If the pilot is lost and desires to know how far he can safely descendhe may drop a number of timing bombs at intervals, as he descends, andwhen he gets as low as he can safely go, based on the ground clearancedetermined from the previously released timing bombs, he may turn 180 toretrace his course at the lowest safe elevations over the area probed bythe released timing bombs in an attempt to get under the ceiling.

In a third practice of my invention represented by Fig. 8, I contemplateemploying a relatively simple continuously running springwound clock |40in combination with means for automatically photographing the clock asrequired for the calculation of falling periods of successive timingbomlbs. The arrangement is simpler than the arrangement of Fig. 6, inthat the clock is not complicated by any electrical controls whatsoever.

vIn Fig. 8 a release mechanism generally designated |4| includes a hook|42 on a latch-lever |43, the hook being adapted to releasably retainone of the previously described timing bombs |0. A spring |44 connectedto the latch-lever .|43 urges the latch lever to the open orbombreleasing position of the lever, but is not strong enough to preventa solenoid |45 from holding the latch-lever in closed position whenevera latching circuit represented by wires |46 and |41 is closed. Thelatching circuit is energized by a is equipped with a. shutter |6| thatis operated by a solenoid |62, the shutter being adapted to aspring-actuated'motor |63 for winding nlm bei tween exposures by theshutter |6 and th motor suitable battery |40 and is controlled by alatching switch |50.

Associated with the'clock |40 is a masking solenoid |52 corresponding tothe previously described masking solenoid |28. 'Ihe masking solenoid |52operates in a retarded manner by virtue of an\apertured bellows |53 andcarriesv a mask |54 that drops in front of a white plate |55'when thesolenoid is deenergized. The masking solenoid |52 is placed in shuntacross the latching circuit by a wire |56v connected to the wire |46 anda wire |51 connected to the wire In prcper position for photographingthe clock |40 is a camera |60 that corresponds to and is operated in thesame electrical manner as the previously described clock 9|. The camera|60 |63 is controlled by a lm solenoid |64, the film solenoid beingretarded in its movement by an apertured bellows |65. In the same manneras previously described, the lm in the camera |60 is advanced for a newexposure whenever the lm solenoid |64 is either energized ordeenergized, the apertured bellows |65 causing the ilm movement to lagbehind operation of the shutter solenoid |62.

In the arrangement represented by Fig. 8 Icontemplate employing a timingcircuit in which the two solenoids |62 and |64 are in parallel and inwhich a switch member |66 and a second switch member |61 are in seriesto control the operation of the two solenoids. Such a timing circuit mayinclude a wire |10 from one terminalof the battery |49 branching to eachof the solenoids |62 and |64, and a second wire |1| from the otherterminal of the battery branching to the two solenoids, the wire |10being broken by the switch member 66 and the wire 1| being broken by theswitch member |61. The switch member |66 ispart yof a relay |12 in theoutput circuit of a radio receiver on the aircraft,4 which outputcircuit is represented by wires |13 and |14. The switch member |66 opensand closes with energizatio'n and deenergization respectively of therelay |12. The switch member |61 is part of a second relay |15 that isplaced in shuntacrossY the latching circuit by two wires |16 and |11connected respectively to wires |46 and |41. The switch member |61 seeksa closed position when the latch circuit is open, but is moved to openposition by energization of the relay |15.

In the normal procedure, the operator places a bomb in engagement withthe hook |42 of the release mechanism |4| and closes the latching switch|50 to energize the latch solenoid |45 and thereby hold the hook inengaging position to retain the bomb. Since the masking solenoid 52 isin parallel with the latch solenoid |45, closing the latching switch |50causes the mask |54 to be shifted to its upper position. It iscontemplated that the transmitter in the bomb will be deenergizedprior'to engagement by the release mechanism,V and that the transmitterof the bomb will be energized immediately after the timing bomb is soengaged. In such a procedure the timing circuit of Fig. 8 is open priorto the placing of the timing bomb in the release mechanism, because atsuch time the relay |12 of the receiver circuit is deenergized and theswitch member |66 of the timing circuit is in open position. Theengagement of the timing bomb while the output circuit of the receiveris deenergized causes the second switch member |61 to open, the positionof the various elements then being as indicated in Fig. 8. When thetransmitter is thereafter energized, the switch member |66 in the relay|12 is drawn to closed position, but closing of the switch member |66under such circumstances does not close the timing circuit becausev theswitch member |61 remains open so long as the latching circuit isenergized. After the switch member |66 closes, the timing bomb maybereleased at the operators vdiscretion by simply opening the latch switch|50 to break the latching circuit and thereby permit the spring |44 toswing the latch lever |13 to open position.

As soon as the latch circuit is broken to release the timing bomb, thedeenergization of the relay permits the switch member |61 to move to aclosed position, thereby closing the timing circuit. Closing of thetiming circuit causes instantaneous operation of the camera shutter anddelayed operation of the film advancing mechanism. The breaking of thelatch circuit'deenergives the solenoid |52 to permit the mask |54 tomove downward, but the downward movement of the mask is delayed by thebellows |53 so that tne operation of the camera. shutter in response toclosing of the timing circuit results in a picture of the clock whilethe mask is still at its upper position. When the radio transmissionterminates upon impact of the timing bomb with the-ground, the relay |12is deenergized whereupon the switch member |66 moves to open position tobreak the timing circuit. Breaking of the timing circuit causesinstantaneous operation of the shutter ISI, delayed .operation of thefilm advancing mechanism |63, and retarded fall of the mask |54.

It is apparent that a picture of the clock |40 representing thebeginning of a falling period will show the mask |54 in its upperposition and a subsequent picture at the end of a falling period willshow the mask in its lower position. A comparison of the two clockreadings will represent L the duration of the falling period. Preferablythe clock has a dial, as previously described, showing a time -span ofone hundred seconds with one hand indicating seconds and another handindicating hundredths of a second.

The selected practices and forms of my invention described herein indetail for the purpose of disclosure and to illustrate the principlesinvolved will suggest to those skilled in the art various changes,modifications, and substitutions that do not depart from my underlyinginventive concept, and I reserve the right to all such changes,modiflcations, and substitutions that properly come within the scope ofmy appended claims.

I claim as my invention:

l. A method of obtaining from an aircraft an aerial photograph that maybe used for obtaining measurements of terrain below characterized by theemployment of an object adapted to produce a photogenically perceptiblesignal upon impact with the ground. sai'd method including the steps of:dropping said object to the earth from the aircraft; timing the durationof the fall of said object; photographing the terrain from the aircraftafter said impact to obtain an z image of the terrain including saidphotogenically perceptible signal; computing from the duration of saidfall the vertical distance from the point at which the photograph ismade to the point of impact of said object on the terrain; and computingthe scale of the photograph from said vertical distance and the focallength of the camera.

2. A method of obtaining from an aircraft measurements of the terrainbelow characterized by the employment of a camera and an object adaptedto produce a photogenically perceptible signal upon impact with theground, said method including the steps of: dropping said object to theearth from the aircraft; timing the duration of the fall of said object;photographing the terrain from the aircraft after said impact to obtainan image of the terrain including said photogenically perceptiblesignal; computing from the duration of said fall the vertical distancefrom the point at which the photograph-is made to the point of impact ofsaid object on the terrain;

computing the scale of the photograph from said vertical distance andthe focal length of the camera; and measuring distances on thephotograph in accord with said scale.

3. An aerial mapping method characterized by the employment of an objectadapted to produce a photogenically perceptible signal when droppedagainst the earth, and the employment of a camera of known focal length,said method including the steps of: dropping the object to theunderlying terrain from an elevated level; timing the duration of thefall of said object; computing from the duration of said fall thevertical distance from said level to the point of impact of said objectwith the terrain; employing said camera at spaced photographic pointssubstantially at said elevated level to obtain two successivephotographs of the terrain overlapping in the region of the impact ofsaid object with the terrain and including an image of the signalproduced by said impact: computing the scale of said photographs fromsaid vertical distance and said focal length; forming a stereoscopicimage of the terrain of known scale by employing said two photographs ata spatial relationship corresponding to the spatial relationship of saidphotographing points at said elevated level; and measuringdistances onsaid stereoscopic image in accord with said known scale.

4. An aerial mapping method characterized by the employment of an objectadapted to produce a photogenically perceptible signal when drippedagainst the earth, and the employment of a camera of known focal length,said method including the steps of: dropping the object to theunderlying terrain from an elevated level; timing the duration of thefall of said object; computing from the duration of said fall thevertical distance from said level to the point of impact of said objectwith the terrain; employing said camera at spaced photographic pointssubstantially at said elevated level to obtain two successivephotographs of the terrain overlapping in the region of the impact ofsaid object with the terrain and including an image of the signalproduced by said impact; computing the scale of said photographs fromsaid vertical distance and said focal length; forming a steroscopicimage of the terrain of known scale by employing said two photographs ata spatial relationship corresponding to the spatial relationship of saidphotographing points at said elevated level; and measuring the altitudeof other points on the terrain relative to said impact point bymeasuring in said stereoscopic image vertical distances of said otherpoints from the vertical image in accord with said known scale.

5. A method of obtaining from an aircraft an aerial photograph that maybe used for obtaining measurements of terrain below characterized by theemployment of a time-indicating means andan object adapted to produce aphotogenically perceptible signal upon impact with the ground, saidmethod including the steps of dropping said object to the earth fromsaid aircraft; photographing said time-indicating means in the course 0fthe fall of said object to obtain a record of the duration of the fall;photographing the terrain from the aircraft after impact of said objectto obtain an aerial photograph of the terrain including said signal;computing from the duration of said fall the vertical distance from theaircraft to the point of impact of said objectfand computing the scaleof the terrain photograph from said vertical distance and the focallength at which the terrain photograph is taken.

6. A method of obtaining from anaircraft an aerial photograph of terrainthat may be used for obtaining measurements on the terrain characterizedby the employment of an impact-explosive bomb having a knownfree-falling characteristic, said method including the steps of:dropping said bomb to the earth from said aircraft; timing the durationof the fall of the bomb;

photographing the terrain from the aircraft after said impact to obtainan image of the terrain including an image of the explosion products ofthe bomb to identify the point of impact; computing the verticaldistance of said fall from the duration of the fall and saidfree-falling char- "acteristic of the bomb; and computing the scale ofsaid photograph from said vertical distance and the focal length atwhich the photograph l0 is taken.

LEON T. ELIEL.

CERTIFICATE oF CORRECTION. A patent No. 2,521,902. June 15, 1914.5.

' LECN T. ELIELL It is hereby Certified that error appears in theprinted specification of the above numbered patent requiring correotionas follows: Page 2, first column, llne 59", for my read may; page 5,second column, line 15, strike out "it'g page h., first column, line 9,for *set* read --sea--g page 7, first column, line 9-1, for'deenergives" read eianerg'izes-;v and second column, line 5b., cl'amLL', for' *drpped* rand "dropped-m; and

'that the said Letters Patent should be read with this correction tperexthat the same may Conform to the record of the case in th'e Patent:Office.

signed and sealed-thu 27m day or July, A. D. 19H5.

Henry Van Arsdale, (Seal) Acting Commissionervof Patents.

l, v CERTIFICATE op coRREcTIoN. Patent No. 2,21,902v. e I June 15, 1%5.

' LE'oN T. ELIEL It is hereby certified that error appears in theprintedspecification of the above numbered patent requiring correctionas follows: Page 2, first column, line 59", for "my" read -may; page 5,'second column, line 15, strike out *it"; page 11 first column, line 9,for "set" rend sea1 page "I, first column, line 9-1, for 'deenergives"read --deenerg'izea--q' and second column, line 51|., clim 14.', for`dripped" read -dropped; and

'that the said Lettere Patent lould be read with this correction thereinthat the same may conform to the record of the case in the PatentOffice.

signed and sealedthia 27th day or July, A. D. 15145.

Henry Van Arsdale, (Seal) Acting Commisionerof Patents.

